\Header{try}{Try an Expression Allowing Error Recovery}
\keyword{programming}{try}
\begin{Description}\relax
\code{try} is a wrapper to run an expression that might fail and allow
the user's code to handle error-recovery.\end{Description}
\begin{Usage}
\begin{verbatim}
try(expr, silent = FALSE)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{expr}] an \R{} expression to try.
\item[\code{silent}] logical: should the report of error messages be suppressed?
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
\code{try} evaluates an expression and traps any errors that occur
during the evaluation.  \code{try} establishes a handler for
errors that uses the default error handling protocol. It also
establishes a \code{tryRestart} restart that can be used by
\code{invokeRestart}.\end{Details}
\begin{Value}
The value of the expression if \code{expr} is evaluated without error,
but an invisible object of class \code{"try-error"} containing the
error message if it fails. The normal error handling will print the
same message unless \code{options("show.error.messages")} is false or
the call includes \code{silent = TRUE}.\end{Value}
\begin{SeeAlso}\relax
\code{\Link{options}} for setting error handlers and suppressing the
printing of error messages;
\code{\Link{geterrmessage}} for retrieving the last error message.
\code{tryCatch} provides another means of catching and handling
errors.\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
## this example will not work correctly in example(try), but
## it does work correctly if pasted in
options(show.error.messages = FALSE)
try(log("a"))
print(.Last.value)
options(show.error.messages = TRUE)

## alternatively,
print(try(log("a"), TRUE))

## run a simulation, keep only the results that worked.
set.seed(123)
x <- rnorm(50)
doit <- function(x)
{
    x <- sample(x, replace=TRUE)
    if(length(unique(x)) > 30) mean(x)
    else stop("too few unique points")
}
## alternative 1
res <- lapply(1:100, function(i) try(doit(x), TRUE))
## alternative 2
## Don't run: 
res <- vector("list", 100)
for(i in 1:100) res[[i]] <- try(doit(x), TRUE)
## End Don't run
unlist(res[sapply(res, function(x) !inherits(x, "try-error"))])
\end{ExampleCode}
\end{Examples}

